1. Field of the Invention
The present invention relates to a piezoelectric vibrator made from a piezoelectric material such as a piezoelectric ceramic, quartz or lithium niobate, which can be used as a component of an oscillator, a ceramic filter or the like, and in particular to a holding structure of the piezoelectric vibrator and a manufacturing method thereof.
2. Description of the Prior Art
A piezoelectric vibrator is used as a mechanical filter, an oscillator, an actuator or the like and is made from a piezoelectric material such as a piezoelectric ceramic, quartz or lithium niobate. A suitable vibration mode such as bending vibration or extensional vibration is selected according to a frequency range to be used. When an alternating electric voltage is applied to a piezoelectric vibrator, it is vibrated due to the piezoelectric effect with a low vibration loss. Such a piezoelectric vibrator has large mechanical quality factors Q.sub.m and large coupling factors. Therefore, a ceramic filter can have a wider range and a vibrator has a better frequency precision than an LC filter and a vibrator using a capacitor and a coil.
In order to utilize such features of a piezoelectric vibrator, it is important to support or fix a piezoelectric vibrator correctly and surely at a node where the vibration amplitude becomes zero so as not to suppress vibrations generated in the piezoelectric vibrator in a holding structure thereof.
A node in a piezoelectric vibrator is a point or a line without zero area. Therefore, it is desirable that the holding structure holds a piezoelectric vibrator at a point or at a line. However, it is practically impossible to hold a piezoelectric vibrator at a point or at a line. Further, it is also required to be stably held. Thus, a piezoelectric vibrator is held in practice at a point-like area or at a linear area by sacrificing the characteristics thereof somewhat.
Previously, a piezoelectric vibrator was supported or fixed around a node with the use of springs. However, this technique the following problems: (1) It is difficult to set positions for supporting or fixing a piezoelectric vibrator correctly around a node. (2) If a piezoelectric vibrator is supported or fixed strongly or a strong force is applied to the piezoelectric vibrator for supporting or fixing it surely, the characteristics of the piezoelectric vibrator are deteriorated or the piezoelectric vibrator is destroyed mechanically. (3) It is needed to support or fix a piezoelectric vibrator on a certain area in order to hold it at a particular posture. However, because a node of the piezoelectric vibrator is a point or a line with no area, this means that vibrating portions in the piezovibrator are held or fixed. Therefore, vibrations are suppressed and characteristics of the piezoelectric vibrator are deteriorated.
FIG. 1 illustrates a concept of the holding of a piezoelectric vibrator 1 at points. Driving electrodes 2 are formed on two opposing principal planes of a piezoelectric vibrator 1, a square plate made of a piezoelectric material. When an alternating voltage is applied to the driving electrodes 2, extensional vibrations of the square plate are generated. Because the vibrations have a node point at a center of the square plate, the piezoelectric vibrator 1 is held by holders 3 to interpose it between two sides with an applied pressure. A top of the holder 3 has a flat region with a certain area, and the piezoelectric vibrator 1 is held stably by abutting it to the flat regions of the holder 3 at two principal planes. Further, the elastic property of the holder 3 buffers vibrations propagated from the outside.
FIG. 2 illustrates the concept of holding a piezoelectric vibrator 1 at a line. Driving electrodes 5 are formed on two opposing principal planes of a piezoelectric vibrator 4, a rectangular plate made of a piezoelectric material. When an alternating voltage is applied to the driving electrodes 5, longitudinal vibrations of the rectangular plate are generated having main displacements along longitudinal direction of the rectangular plate. Because the vibrations have a node line at a center line of the rectangular plate, the piezoelectric vibrator 4 is held at two principal planes with an applied pressure by interposing it between two holders 6. A top of the holder 6 has a flat region with a certain area, and the piezoelectric vibrator 4 is held stably by abutting the flat regions thereto at two sides. Further, the elastic property of the holder 6 buffers vibrations propagated from the outside.
As described above, prior art holding structures hold a piezoelectric vibrator directly with holders at point-like areas or at line-like areas having a certain area. Therefore, if the contact areas for holding are decreased, the holding becomes unstable. Further, when a shock is given, for example by the vibrator falling to the ground, the posture or position is liable to be changed the and this causes vibration characteristics to greatly change. On the other hand, if the holding area is increased, a part of the vibrating region is pressed and both mechanical quality factors and coupling factors decrease to deteriorate the vibration characteristics. Because a piezoelectric vibrator is held directly with holders, a contact area between the vibrator and the holder and a holding position of the holders vary for each piezoelectric vibrator. Then it is also a problem that the characteristic of the piezoelectric vibrator scatters. To sum up, it is difficult to hold a piezoelectric vibrator stably and reliably without affecting the vibrator characteristics.
In order to solve the above-mentioned problems, it has been proposed to form protrusions at positions for supporting or fixing a piezoelectric vibrator and to support it at the protrusions. This technique has an advantage in that if protrusions can be formed correctly at a node, the position for supporting or fixing can be set surely around the node. Previously, such protrusions were formed with a hard soldering process, a soldering process or a plating for a metallic material and with a printing process for an organic electrically conductive paint. The protrusions were made of an electrically conducting material because electrical leads are connected at the protrusions.
Japanese Utility Model laid open Publication 20422/1993 proposes a holding structure for a piezoelectric vibrator wherein protrusions made of electrically conductive rectangular rubber are formed around a node point of vibrations of a piezoelectric vibrator and the piezoelectric vibrator is held at the protrusions. However, this holding structure has a problem in that the protrusions are deformed by the pressure applied for holding. Then a contact area between the vibrator and the holders is not constant among piezoelectric vibrators, and the characteristics of the piezoelectric vibrator varies greatly.
Japanese Utility Model laid open Publication 23617/1993 proposes a holding structure for a piezoelectric vibrator wherein metallic protrusions are formed around a node of vibrations of a piezoelectric vibrator and the piezoelectric vibrator is held at the protrusions. It is proposed to produce protrusions by soldering, welding, melting, adhesion or the like. However, when the protrusions are produced with a technique using heating, the temperature of the piezoelectric vibrator is liable to be heated to deteriorate the characteristics of the piezoelectric vibrator. The deterioration is especially large for a piezoelectric vibrator made of a piezoelectric ceramic having a low Curie temperature. Generally, the Curie temperature of a piezoelectric ceramic is 250.degree.-350.degree. C., and when a piezoelectric vibrator is heated for a long time at say 125.degree. C. or above, the characteristics are greatly deteriorated. In an example, a piezoelectric ceramic of 300.degree. C. of Curie temperature is heated for an hour, a piezoelectric constant is found not to change for heating at 140.degree. C. or lower, but to change for heating above it. It is also found that the piezoelectric constant decreases in a shorter time with an increasing heating temperature. If the metallic protrusions are formed with adhesion without heating it is a problem in that an adhesive agent extends outside the contact areas. This also greatly scatters and deteriorates the characteristics of the piezoelectric vibrator.